The present invention relates to a noise reduction treatment for an aircraft cabin, and more particularly to an acoustic absorption system which is specifically tailored to particular aircraft types.
Noise develops in an aircraft cabin from several sources. The most common sources are internally or externally mounted moving components, such as a transmission, engine or rotor system. Another source of cabin noise is air flows over various aircraft fuselage components. These components generate vibrations in the aircraft that propagate through the airframe and radiate into the cabin.
Noise may be a particular problem in rotary wing aircraft cabins since the rotor and transmission systems produce a significant amount of vibration directly into the airframe structure. One main noise problem in helicopter cabins is mid to high frequency gear whine noise from the main transmission. This results in cabin noise vibrations typically from about 350 Hz through 4,000 Hz. In contrast noise vibrations from the main and tail rotor sources are in the 20 Hz to 125 Hz range and are attenuated by up to 40+dB by the response of the human ear.
Aircraft cabin interiors are generally designed to maintain aircraft interior noise below a certain level predetermined by competitive pressures in the marketplace. For example, executive transport rotary wing aircraft typically provide a design average noise level limit with the environmental control system (fans, vent air and cooling/heating system) turned off of approximately 75 dB SIL4. The SIL4 (Speech Interference Level 4) noise measurement metric is the arithmetic average of the sound pressure levels in the 500, 1000, 2000 and 4000 Hz octave bands. It rates steady noise according to interference with conversation between two people.
Various acoustic absorption systems have been provided to reduce noise levels within the cabin to below desired SIL4 values. Each of these systems, however, is typically manufactured independent of an aircraft type. Although quite effective in generally, particular aircraft types produce particular noise signatures which may not be amenable to a single generic acoustic absorption system.
Accordingly, it is desirable to provide a lightweight, acoustic absorption system which may be specifically tailored to particular aircraft types.